The present invention generally relates to code division multiple access (CDMA) communication systems employing time division duplex (TDD). More specifically, the present invention is a TDD system which implements a novel method for handing over a mobile terminal between two base stations.
A Universal Mobile Telecommunications System (UMTS) network architecture as shown in FIG. 1 includes a core network (CN), a UMTS Terrestrial Radio Access Network (UTRAN) and a User Equipment (UE). The two general interfaces are the Iu interface, which is coupled between the UTRAN and the core network, and the radio interface Uu, which is coupled between the UTRAN and the UE. The UTRAN consists of several Radio Network Subsystems (RNS). They can be interconnected by the Iur interface. This interconnection allows core network independent procedures between different RNSs. The RNS is further divided into the Radio Network Controller (RNC) and several base stations (Node-Bs). The base stations are connected to the RNC by the Iub interface. One base station can serve one or multiple cells, and typically serves a plurality of UEs. Although the UTRAN supports both FDD mode and TDD mode on the radio interface, the present invention is related only to the TDD mode.
In the current UMTS TDD systems, the sole criterion for handover, whether intra- or inter-frequency, is the path loss difference between the present base station, (i.e. the “serving” base station), and a target base station. This situation is shown in FIG. 2. The mobile user equipment (UE) is shown receiving signals from two base stations: 1) the serving base station BSser; and 2) the target base station BStar. The UE receives the physical channel that carries the broadcast channel BCHser from the serving base station BSser and the physical channel that carries the broadcast channel BCHtar from the target base station BStar. The UE measures the strength of the channels BCHser, BCHtar. When the BCHtar from the target cell is sufficiently stronger than the channel BCHser from the serving cell, the measurements are transmitted to the RNC, which determines whether or not to initiate a handover. Alternatively, measurements can periodically be signaled to the RNC for the purpose.
The current procedure 10 carried out by a prior art UMTS TDD communication system for determining whether to commence handover can be generally explained with reference to FIG. 3. The UE receives the physical channel that carries the broadcast channel (BCHser) from the serving base station BSser (step 12) and calculates its strength. The UE also receives the physical channel that carries the broadcast channel (BCHtar) from the target base station BStar (step 16) and also calculates its strength. Periodically, or depending on the relative signal strengths, the information is signaled to the RNC which determines the BCHser path loss (step 14) and the BCHtar path loss (step 18).
It is then determined whether the BCHser path loss is greater than the BCHtar pathloss (step 20). If it is not, no further action is taken. If, however, the BCHser path loss is greater than the BCHtar path loss as determined by step 20, the handover to the target base station BStar (step 22) is typically commenced.
Typically, the values measured at steps 12 and 16 are transmitted to the RNC, and steps 14, 18 and 20 are performed at the RNC.
Although this example illustrates a single target cell, the same is true for a multiple of target cells of which the UE is aware, either by detecting their presence or by having received their parameters from the serving cell.
In TDD systems that use multi-user detecting (MUD) receivers, the interference measured in the serving cell is different from other cells. A prerequisite to receiving data in any cell is the ability to decode the BCH channel in the cell. Due to the low spreading factor used in TDD this may be difficult, particularly at cell edge. Therefore it would be desirable to ensure that BCH reception is possible in the target cell prior to the handover.
In addition to the path loss, BCH reception depends on the interference in the slot and prior knowledge of its level is necessary to determine its likelihood. This is particularly true in small cells where the interference level is typically higher, and the interference is also different from cell to cell and UE to UE. Observing the interference in the serving cell will typically provide no information about the interference in the target cell because in the case of a MUD receiver, different slots or different frequencies may be used. Thus it would be desirable to measure the interference in the slot which carries the BCH in the target cell.